Apparatus for enhancing image contrast using intensity filtration

ABSTRACT

An intensity filter for deep UV lithography enhances contrast and also therefore increases the resolution of patterned images by passing only intensities that fall within a specific minimum threshold value, resulting in a more exact aerial image replicating the mask image. This device is a different approach to contrast enhancement that is distinguished from previous methods by eliminating the need for an extra layer of contrast enhancement on top of the resist, thereby reducing the number of processing steps in semiconductor fabrication.

BACKGROUND OF THE INVENTION

1. 1. Technical Field

2. The present invention relates generally to the field of semiconductorfabrication and in particular to photolithographic processes used in themanufacture of semiconductor devices. Still more particular, the presentinvention relates to a method and apparatus for reducing degradation inthe contrast of resist images produced from a mask image.

3. 2. Description of the Related Art

4. Microcircuit fabrication requires that precisely controlledquantities of impurities be introduced into very small regions of asubstrate. These regions are subsequently interconnected to createdcomponents and very large scale integration (VLSI) circuits. Thepatterns that define these regions are created by lithographicprocesses. Typically, photoresist materials are spun onto a wafersubstrate. Then, the photoresist is selectively exposed to radiation,such as ultraviolet light, electrons or x-rays. An exposure tool and amask are used to cause the desired exposure of the photoresist. Thepatterns in the resist are formed when the wafer under goes adevelopment step. The areas of photoresist remaining after developmentof the photoresist protect the covered regions of the substrate duringintroduction of impurities or during etching of exposed regions of thesubstrate.

5. In the art of deep ultraviolet microlithography, much demand astechnology develops is placed upon increased resolution, tighterplacement of features in proximity to one another, and smaller butwell-defined features. In order to continue meet increasing demands,many methods and devices have been developed and tested. One suchconcept is the control of images exposed onto a resist laver byenhancing the contrast of the said images.

6. In the process of patterning an image on a laver of resist on awafer, light passes through openings on a mask, then through a lenschamber, and finally onto the wafer that is coated with resist. A “mask”is a pattern tool, which contains patterns that can be transferred to anentire wafer or to another mask in one exposure. At the resist level,portions of the resist that coincide with the image pattern are exposedand then developed to reproduce the same original image in a greatlyreduced form. When light passes though the mask, sub harmonics areintroduced into the intensity distribution of the aerial image. Thesesub harmonics cause the aerial image formed at the resist layer to be aninexact replica of the mask image. The resist image formed from the rawaerial image tends to have degraded contrast using presently availableprocesses and equipment due to coinciding harmonics that are transmittedin addition to the desired zeroeth order element. Therefore, the samesub harmonics also limit the maximum resolution allowed in the finalimage. In order to reduce the distortions, previous methods improved thecontrast by relying on the use of an extra layer that is formulatedspecifically for contrast enhancement coated on top of the resist layer.

7. However, such methods lengthen the processing of semiconductors byrequiring additional process steps to place a coat of a contrastenhancement layer on top of the resist layer.

8. Therefore it would be advantageous to have a system and a method forenhancing contrast with minimal additional process steps.

SUMMARY OF THE INVENTION

9. The present invention provides an intensity filter for deepultraviolet lithography for enhancing contrast. The intensity filterfilters light having various intensities. The intensity filter includesa first material and a second material in which these two materialsinterface in such a wav that only specific intensities are passedthrough. The first material is non-linear in nature and has a refractiveindex that changes at high intensities, but has a constant refractiveindex substantially equivalent to the second material at a selectedintensity. The second material has a constant refractive indexirregardless of varying levels of intensity, at intensities lower than aspecific minimum threshold. The filter also may include a coating thatwill phase shift the exiting filtered light 180-degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

10. The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself however, as well as apreferred mode of use, further objects and advantages thereof, will bestbe understood by reference to the following detailed description of anillustrative embodiment when read in conjunction with the accompanyingdrawings, wherein:

11.FIG. 1 is a schematic illustrating the configuration of the inventionitself and its relative positioning within a simplified exposure systemin deep ultraviolet lithography;

12.FIG. 2 is a schematic demonstrating the effects of the invention onvarious degreed orders of light at different stages of the process flowthrough and resultant of the present invention;

13.FIG. 3 is a detraction pattern graph;

14.FIG. 4 is a relative intensity distribution graph;

15.FIG. 5 is a relative intensity distribution graph depicting theresult of filtering the light as it passes through the phase shiftedintensity filter cell in accordance with a preferred embodiment of thepresent invention; and

16.FIG. 6 is a relative intensity distribution graph of light left fromthe filtered light is depicted in accordance with a preferred embodimentof the present invention.

DETAILED DESCRIPTION

17. The present invention provides a method and apparatus for passingonly intensities that fall below a specific minimum threshold. Theminimum threshold value can be set equivalent to the intensity of thezeroeth order harmonic. This value can be derived for the smallestfeature size on the mask. By so doing, the zeroeth order harmonics forall the other features will be greater than the zeroeth order elementfor the minimum feature. Thereby the resulting image will be a“filtered” version of the aerial image consisting only of side lobes.Rather than compensating for the additional harmonics at the resistlevel, the present invention eliminates sub harmonics before theyimprint on the resist itself. Therefore, the intensity filter systemprovided for by the present invention acts as a low pass filter.

18. The filter used in the present invention includes two properties.One property of the filter is that it should not pass light of anintensity above a certain critical value. The other property of thefilter requires that it does pass light of any intensity below thatspecified value. The present invention provides for constructing anintensity filter cell comprised of at least two materials. One desiredmaterial has a linear, or constant, refractive index. The other materialhas the quality of a non-linear or an intensity dependent, refractiveindex. These two materials interact together in such a way that, ifplaced correctly in the form of an enclosed cell, a very high contrastimage will form in the resist after being developed. The two materialsin the depicted example are selected in such a way that the refractiveindices of the materials are equivalent at one or more selectedintensities, such as, low intensities, and the elements are also to beessentially transmissive. In the depicted examples. low intensities aredefined as intensities that are less than the intensity of the firstharmonic of the smallest feature of the mask. A low intensity tvpicallyabout 10 mJ. In the depicted examples, high intensities are defined asintensities equal to or greater than the intensity of the first harmonicof the smallest feature of the mask. High intensities are typicallygreater than 10 mJ. At high intensities, determined by the zeroeth orderharmonic of the minimum feature, the refractive index of the non-linearmaterial, will change substantially enough to cause the combinedelements to have a mismatched refractive index. This mismatch in therefractive indexes between the two materials leads to poor transmissionof light or even complete internal refraction. This event results in theblocking out of the ligh intensity components of the aerial image.

19. In addition to emploving refractive index compatibility to filterout unwanted sub harmonics, the filter of the present invention also mayemploy a coating to enhance contrast. In the depicted example, thecoating has a thickness such that a 180-degree phase shift in the lightis produced. In the depicted example, the thickness of the coating forthe intensity filter cell can be determined by the equation:

t=λ/4,

20. where λ is equal to the wavelength of the incident light. Thefiltered and phased shifted light is then combined with the originallight having an intensity distribution present prior to filtering usingthe intensity filter. The result of this union of the filtered and phaseshifted light and the original light has an aerial image comprising ofonly the principle harmonic of the image. When this image is madeincident on the photo resist layer, the contrast is markedly improved,thereby improving the resolution as well.

21. With reference to the figures and in particular to FIG. 1, a diagramof a photolithography apparatus is depicted in accordance with apreferred embodiment of the present invention. Photolighography system100 receives light 102 at a specifically designed mask 104 from anillumination source (not shown). Mask 104 is used to describe a patternto be exposed on a photo resist layer. Mask 104 is a photomask. Light102 upon passing through the various openings in mask 104 is thendiffracted resulting in sub harmonics being thereby introduced into theintensity distribution of the aerial image. The resultant light thenenters the photolithography lens 106, which reduces the image size tothe appropriate ratio then passes into the beam splitter 108. Beamsplitter 108 causes the light 102 to divide into two light beams 109 and111. Light beam 109 travels along a path created by a light guidingapparatus, which in the depicted example includes mirror reflector 110and mirror reflector 112. Light beam 111 travels through intensityfilter cell 114. The light beam 111 is rejoined with light beam 109 asan unfiltered version of the original light with the additional subharmonics.

22. Light beam 111 is filtered as it passes through intensity filtercell 114, which includes material 116, material 118, anti-reflectivelayer 120, anti-reflective layer 122, phase shifter 124, and highintensity absorber 125. Material 116 is a material that has a refractiveindex that is linear, or constant. Material 118 has a refractive indexthat is non-linear, or intensity dependent, nature. A linear material isa material that has a constant index of refraction in which the index ofrefraction is independent incident intensity, which is described asfollows:

n=n₀

23. wherein n is the index of refraction and n₀ is a constant. Thisconstant is typically selected to correspond to the refractive index ofthe lens element, such as photolithography 106 in FIG. 1. In thedepicted example, a non-linear material is a material that has aconstant of index of refraction at a low intensit, and a index ofrefraction dependent on the intensity of incident light at a lowintensity. The index of refraction of a non-linear material may bedescribed as follows:

n=n₀, E<Ea

n=n₀n₀*E/Ea, E>Ea

24. where E is the intensity of incident light and Ea is the intensityat which the material switches from a constant index of refraction to anintensity dependent refractive index of refraction. An example of anon-linear material is MBBA: N-(p-methoxylbenzylidene)-p-butylaniline.

25. A large class of well-known organic and non-organic non-linearoptical materials exists, wherein these materials each have a refractiveindex that changes with the incident intensity. Each of these materialspossesses one constant index of refraction when (1) the intensities fallbelow a specific critical intensity level. and (2) has a differentrefractive index that is variable when the intensities surpass thatspecific level of intensity. Material 116 and material 118 are chosensuch that the pertinent refractive index of each at specific levels ofintensity reacts in a predetermined manner. Material 116 and material118 are selected such that at low intensities the refractive index ofeach are equivalent to the other index of refraction, and therefore aretransmissive in accordance with a preferred embodiment of the presentinvention. However, the refractive index of non-linear material 118 willchange at high intensities and results in non-identical refractiveindices, which results in poor transmission of light, or ideallycomplete internal refraction. In the depicted example, the highintensity levels are established by the zeroeth order harmonic of theminimum feature of the pattern to be made incident on the resist layer.Examples of suitable linear materials include quartz or fused silica.Examples of some materials that may be used for non-linear opticalmaterial 118 are as follows:

26. MBBA: N-(p-methoxylbenzylidene)-p-butylaniline

27. In the depicted example, positioning of material 116 and material118 within intensity filter cell 114 provides one possibleconfiguration. High intensity absorber 125 may be formed using a glassplate coated with an absorbing film, such as silicon. High intensityabsorber 125 in the depicted example is positioned such that it is incontact with one edge of material 118 as shown in FIG. 1. If the orderof the two optical materials are reversed, i.e., material 118 andmaterial 116 are switched, the absorber would be moved to the oppositeside of intensity filter cell 114, such that it is still in contact withmaterial 118. In the depicted example, the intensity filter cell willhave a dimension at least equal to a reduced image of the mask afterpassing through the lens. These dimensions are typically about 25 mm by25 mm. The filter establishes a filtered variation on the original lightbeam. Material 116 and material 118 are enclosed between anti reflectivelayers 120 and 122. The light exiting intensity filter 114 then passesthrough the phase shifter 124, wherein the phase of the light is shifted180-degrees. The necessary thickness of the coating comprising phaseshifter 124 is determined by the equation

t=λ/4

28. where λ is equal to the wavelength of the incident light.

29. A number of different types of phase shifting materials may be usedfor the coating. For example, chrome and calcium flouride may be used asa coating. Phase shifter 124 is employed to reverse (or to phase shiftby 180 degrees) the intensity distribution coming out of a filter. Whencombined with an unshifted distribution of light in light beam 109, allof the subharmonics except the zeroeth order are removed, as illustratedin FIG. 6 below. When the light exits the intensity filter cell. thelight has specific intensities filtered out and is phase shifted. Theresultant light is then combined with the original distribution of lightsplit beam splitter 108 and is deterred by mirror reflector 110 andmirror reflector 112, thereby the resulting aerial image is inclusive ofonly the principle harmonic of the image. The desired aerial image isthen made incident on the resist layer 126 coating the wafer 128. Theimage exposed onto the resist layer at this point has an enhancedcontrast, and therefore the image also exhibits improved resolution.

30. Referring now to FIG. 2, a diagram detailing the effects of thepresent invention on the different orders of light as it passes througha mask is depicted in accordance with a preferred embodiment of thepresent invention. Light 202 from the illumination source again passesthrough mask 204 only at specific openings set within the opaquematerial in mask 204. Specifically, light passes through opening 206 inmask 204 and are blocked by opaque portions 208 and mask 204. FIG. 3 isa defraction pattern graph. When light 202 passes through opening 206 ofmask 204, light 202 through diffraction, acquires additional andunwanted sub harmonics as depicted in the diffraction pattern graph 300in FIG. 3.

31. With reference now to FIG. 4. a relative intensity distributiongraph is depicted. The light pattern, after diffraction, is againrepresented in the relative intensity distribution graph 400. This graphshows a first order of light 402, a second order of light 404. and athird order of light 406 along with a original zeroeth order of light408. With these undesired harmonics, the aerial image when made incidentupon the resist layer exhibits degradation in the resultant resistimage, thereby limiting the proximity that two neighboring features canbe placed together. When the sub harmonics are not countered, thediffracted aerial images from the two neighboring features willcoincide, which results in a loss of resolution.

32. With reference now to FIG. 5, a relative intensity distributiongraph 500 depicting the result of filtering the light as it passesthrough the phase shifted intensity filter cell is illustrated inaccordance with a preferred embodiment of the present invention. Inrelative intensity distribution graph 500, the zeroeth order of lighthas been removed from the light, and the remaining sub harmonics havebeen phase shifted 180-degrees. As can be seen. the first order of light502, second order of light 504, and third order of light 506 are phaseshifted 180 degrees from those in FIG. 4. When the resulting light isthen combined with the original light pattern, the 180-degree phaseshifted harmonics and their original respective unfiltered andnon-shifted harmonics cancel each other out, leaving only the desiredzeroeth order of light as exhibited in the final intensity distributiongraph 500.

33. With reference now to FIG. 6, a relative intensity distributiongraph 600 of light left from the filtered light is depicted inaccordance with a preferred embodiment of the present invention. Inrelative intensity distribution graph 600, a zeroeth order of lightremains after filtering.

34. The method and apparatus so described can be built in variousmanners and forms depending on the implementation. In the depictedexample material 116 and material 118 must be situated and confinedwithin an enclosed cell that is encapsulated with an anti-reflectivematerial, and the exiting end is covered with a 180-degree phaseshifting compound.

35. Thus with deep ultraviolet lithography and the constant demand forsmaller and closer set features, the present invention provides therequired resolution increases needed to maintain functionality of thecomponents with the semiconductor devices. Thus, the present inventionprovides an improved approach to the concept of contrast enhancement byintroducing an “intensity filtration” system between the resist and thelens, thereby preventing the sub harmonics from travelling to the resistlayer. The present invention provides this advantage by filtering offharmonics in the intensity distribution of the aerial image in a DUVlithography system. By filtering off specific harmonics, the method andapparatus provides an improved contrast in the resist image. Theresulting image will also be free of harmonic sidelobes. Theseimprovements allow two adjacent features to be located closer togetherwithout interference from each other. Without the enhancement incontrast, the same features in the same locations would lose resolutiondue to the fact that the diffracted aerial images of the two featureswould coincide.

36. The description of the preferred embodiment of the present inventionhas been presented for purposes of illustration and description, but isnot limited to be exhaustive or limited to the invention in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. The embodiment was chosen and described inorder to best explain the principles of the invention the practicalapplication to enable others of ordinary skill in the art to understandthe invention for various embodiments with various modifications as aresuited to the particular use contemplated.

What is claimed is:
 1. An apparatus comprising: a lens; a beam splitter,wherein light passing through the lens is split into a first light beamand a second light beam; a filter including: an input, wherein lightpassing thorough the lens is received at the input; a linear materialhaving a first refractive index; a nonlinear material having a secondrefractive index, wherein the second refractive index varies from afirst intensity of light to a second intensity of light; and an output,wherein the first refractive index substantially matches the secondrefractive index at the first intensity of light such that light of thefirst intensity in the first light beam is passed though the filter andwherein the first refractive index and the second refractive index aremismatched at the second intensity of light such that light of thesecond intensity in the first light beam is blocked from passing throughthe filter to the output; and a light guiding apparatus, wherein thelight guiding apparatus receives the second Iight beam and guides thesecond light beam such that the second light beam merges with the firstlight beam at the output of the filter. wherein a merged light beam isformed.
 2. The apparatus of claim 1 , wherein the filter furtherincludes a phase shifter, wherein the phase shifter shifts the phase ofthe light 180 degrees at the output of the filter.
 3. The apparatus ofclaim 1 , wherein the phase shifter us a coating having a thicknessdetermined as follows: t=λ/4, where λ is equal to the wavelength ofincident light.
 4. The apparatus of claim 2 , wherein the filterincludes a first antireflective layer located between the input and thefirst and second materials and a second antireflective material locatedbetween the phase shifter and the first and second materials.
 5. Theapparatus of claim 1 , wherein the first refractive index is selectedusing the following: n=n₀ wherein n is a refractive index and n₀ is aconstant.
 6. The apparatus of claim 5 , wherein the second refractiveindex is selected using the following: n=n₀, E<Ea n=n₀n₀*E/Ea, E>Eawherein n is a refractive index and n₀ is a constant, E is an intensityof incident light, and Ea is an intensity at which a material switchesfrom a constant index of refraction to an intensity dependent refractiveindex of refraction.
 7. The apparatus of claim 1 , wherein the nonlinearmaterial is N-(p-methoxylbenzylidene)-p-butylaniline.
 8. The apparatusof claim 1 fLirther comprising a mask located betveen the lens and alight source.
 9. The apparatus of claim 1 , wherein the light guidingapparatus includes a mirror deflector.
 10. A filter comprising: a firstmaterial; and a second material, wherein the first material and thesecond material pass light intensities only below a selected threshold.11. The filter of claim 10 , wherein the first material is a linearmaterial and the second material is a nonlinear material.
 12. The filterof claim 11 , wherein the linear material has a refractive index that isselected as follows: n=n₀ wherein n is a refractive index and n₀ is aconstant.
 13. The filter of claim 12 , wherein the nonlinear materialhas a refractive index that is selected as follows: n=n₀, E<Ean=n₀n₀*E/Ea, E>Ea wherein n is a refractive index and n₀ is a constant,E is an intensity of incident light, and Ea is an intensity at which amaterial switches from a constant index of refraction to an intensitydependent refractive index of refraction.
 14. The filter of claim 11 ,wherein the nonlinear material isN-(p-methoxylbenzylidene)-p-butylaniline.
 15. The filter of claim 10 ,wherein the filter further includes a phase shifter, wherein the phaseshifter shifts the phase of the light 180 degrees at the output of thefilter.
 16. An intensity filter for enhancing contrast of an imagecreated by passing light through a mask, wherein the described intensityfilter contains: a first material and a second material, wherein thefirst material and second material interface in such a wav that onlyspecific intensities are passed through, wherein the first material isnonlinear in nature and has a refractive index that changes at highintensities but has a constant refractive index equivalent to the secondmaterial and wherein the second material has a constant refractive indexirregardless of varying levels of intensity, at intensities lower than aspecific minimum threshold, wherein contrast of the image is enhanced bythe filter.
 17. The intensity filter of claim 16 , wherein the filterincludes a coating, wherein the coating phase shifts exiting filteredlight by 180 degrees.
 18. A method for forming a photoresist materialduring fabrication of an integrated circuit, the method comprising:providing a masking member; and providing a filter used to filter lighthaving a range of intensity from a first intensity to a secondintensity, wherein the filter includes a first material and a secondmaterial, the first material having a constant refractive index and thesecond material having a intensity dependent refractive index, whereinat the first intensity the constant refractive indent is about the sameas the intensity dependent refractive index and at the second intensitythe intensity dependent refractive index changes such that a refractiveindex mismatch occurs between the first material and the secondmaterial, projecting light through the masking member onto a layer ofphotosensitive material, wherein light of the second intensity isblocked by the filter.
 19. The method of claim 18 , wherein the filterincludes a phase shifter, which produces a 180 degree phase shift inlight filtered by the filter.
 20. A filter used to filter light having arange of intensity from a first intensity to a second intensity, thefilter comprising: a first material and a second material, the firstmaterial having a constant refractive index and the second materialhaving an intensity dependent refractive index wherein at a the firstintensity the constant refractive index is about the same as theintensity refractive index and at the second intensity the intensitydependent refractive index changes such that a refractive index mismatchoccurs between the first material and the second material and whereinlight of the second intensity is blocked bv the filter from beingprojected through the masking member onto a layer of photosensitivematerial.
 21. The filter of claim 20 , wherein the first material has arefractive index that is selected as follows: n=n₀ wherein n is arefractive index and n₀ is a constant.
 22. The filter of claim 21 ,wherein the second material has a refractive index that is selected asfollows: n=n₀, E<Ea n=n₀n₀*E/Ea, E>Ea wherein n is a refractive indexand n₀ is a constant, E is an intensity of incident light, and Ea is anintensity at which a material switches from a constant index ofrefraction to an intensity dependent refractive index of refraction. 23.The filter of claim 22 , wherein the second material isN-(p-methoxylbenzylidene)-p-butylaniline.
 24. The filter of claim 20 ,wherein the filter further includes a phase shifter, wherein the phaseshifter shifts the phase of the light projected onto the masking memberby 180 degrees.